feat(draft-docs): add Explanation content type support

[hey-amelia]

Summary

Add support for generating Explanation-style documentation to the /beagle:draft-docs command.

Context

The draft-docs command currently supports Reference and How-To content types. Explanations are understanding-oriented documentation that deepen conceptual knowledge.

Requirements

• Create skills/mintlify-explanation-docs/SKILL.md with explanation-specific guidance
• Update draft-docs.md command to detect/offer Explanation type
• Explanation characteristics (from Diátaxis):
  • Deepens conceptual understanding
  • Theoretical, potentially opinionated
  • Provides design decisions and constraints
  • Acknowledges alternative approaches
  • Connects to broader product/industry concepts

Related

• Design doc: docs/plans/2025-12-25-draft-docs-command-design.md
• Parent issue: Replace VitePress docs with Mintlify for v1.0.0 release amelia#142